In recent years, an FMCW (Frequency Modulated Continuous Wave) type radar device is used for detecting an object and measuring a distance (refer to U.S. Pat. No. 6,040,796). The radar device using a detecting method such as the FMCW method transmits a radar wave to detect, for example, a relative speed and/or a distance to a preceding vehicle. Frequency modulated radar wave used in this type of device is transmitted to determine how many objects exist in a detecting area and in which direction they are lying. Direction determination is conducted by using a method such as Beamformer method, MUSIC (Multiple Signal Classification) method, or ESPRIT method. The MUSIC method and the ESPRIT method yield a result having a higher resolution of an incident angle in an angle determination process. These methods yield the high resolution when the number of incoming waves is correctly estimated in the angle determination process.
Estimation of the number of incoming waves is conducted by using a method such as AIC (Akaike Information Criteria) or MDL (Minimum Description Length). In these methods, the incoming wave number is estimated by collecting data for plural times to evaluate the dispersion of the data.
Estimation of the number of waves can also be conducted by using a threshold that is set as a value for distinguishing a signal from a noise in a calculated eigenvalue. A Japanese Patent Document JP-A-2000-121716 discloses an estimation process of the incoming wave number by using the MUSIC method with the threshold.
The estimation process using the MUSIC method with the threshold is described with reference to FIG. 13.
FIG. 13 shows an illustration of a situation that an adaptive antenna having a plurality of antennas in an array at even intervals is receiving an incoming wave on, for example, an automobile. In this case, each antenna in the array receives one of the incoming waves coming at a same incident angle because a distance to a source of the wave is substantially longer than the interval between the antennas. A phase of each incoming wave differs depending on the incident angle.
A weight for minimizing an output of each incoming wave is calculated in the following manner when Pout represents an output voltage of each incoming wave and a vector xi(t) (i:1 to K, K: number of antennas) represents a signal of each incoming wave from each antenna.
                              W          min                ⁡                  (                      Pout            =                                          1                2                            ⁢                              W                H                            ⁢                              R                XX                            ⁢              W                                )                                    [                  Formula          ⁢                                          ⁢          1                ]                            RXX in the formula 1 represents a self-correlation matrix of xi(t). W represents a weight and WH represents a conjugate conversion matrix of W. That is, WH W equals 1. A following value λ (an eigenvalue) is derived by using Lagrange multiplier in Formula 2, and the Formula 2 is converted to Formula 3 when both sides are multiplied by WH.RXXW=λW  [Formula 2]WHRXXW=λWHW=λ  [Formula 3]        
A value of λ takes a value of 2. That is, the output voltage Pout takes a value of 1 when the Pout is weighted by W. In other words, Pout can be regarded as an output signal of a noise when a null point of directivity (zero point) is set at the incident angle of the incoming wave and W is calculated for minimizing the output voltage Pout of the incoming wave coming at the incident angle. This is because all of the output signals of the incoming waves are cancelled in the above-described manner when the value of λ takes a value of 1. The value of λ takes a different value, i.e., different from 1, when the null point of directivity can not be set at the incident angle of the incoming wave. Therefore, the value of λ that represents the eigenvalue of the self-correlation matrix Rxx fulfills a following relationship.λ1≧λ2≧λ3≧ . . . ≧L>λL+1= . . . =λK=σ2  [Formula 4]
In the Formula 4, K represents the number of antenna, and σ2 represents a heat noise voltage.
The relationship shown in Formula 4 indicates that the eigenvalues of the self-correlation function RXX can be used to estimate the number (L) of incoming waves based on the number of eigenvalues greater than the heat noise voltage σ2. That is, a threshold λTH is set to a value between the λL and λL+1, and the threshold λTH is used to estimate the number (L) of incoming waves. Further, the eigenvalue λ greater than the threshold value λTH is regarded to belong to a “noise space” in which all signals of the incoming waves are cancelled, and the eigenvalue λ smaller than the threshold value λTH is regarded to belong to a “signal space” in which at least one signal of the incoming waves is not cancelled. Therefore, the incident angle of the incoming wave is calculated in the “noise space.” This is because all signals of the incoming waves can be cancelled in the “noise space.”
The methods described above are equally characterized in that accuracy of estimation of the incoming wave number increases when the number of collected data is larger and SN ratio (signal noise ratio) in the receiving signal is greater. In other words, the estimation is not accurate when the amount of data is not sufficient.
However, sufficient amount of data can not be collected when an object of detection and the radar device are moving relatively fast toward each other, or processing speed of the radar device is not sufficient. Further, the SN ratio of the signal in the radar device suffers an influence of a multi-path signal problem when the automobile having the radar device is surrounded by a plurality of radar wave reflecting objects. In this kind of situation, the radar device in the automobile using the MUSIC method or the ESPRIT method cannot avoid a false detection of the object or an erroneous operation because the estimation of the incoming wave number is not accurate.